The present invention relates to master/slave clock systems, and more particularly to an electronic module that allows a digital clock to be run and reset to the correct time of day on a two-wire system.
Many systems are in use for transmitting time information from a central station including a master clock to one or more peripheral stations including slave clocks. Master/slave clock systems are those in which a plurality of slave clocks are distributed throughout a given area, but are all controlled from a single master clock or controller. Typically, each slave clock has its own timing mechanism, but responds to the master clock for purposes of setting, synchronizing, and the like. Master/slave clock systems are useful in various applications such as schools, hospitals, or airports where a large number of clocks are distributed throughout the facility, and the master clock synchronizes all of the clocks so every clock tells the same time.
One type of master/slave clock system is generally characterized by transmitting a pulse once each minute from the master clock to the slave clocks. A disadvantage of this type of system is the requirement for manual resetting of the slave clocks when a power failure occurs for the master clock and its associated circuitry. In a large building having many slave clocks, such as a hotel, school, or certain office buildings, manual resetting of the slave clocks can be a problem.
In another type of these systems, the time indication at the master clock is transmitted via a multi-lead cable to the slave clocks. One of the leads is assigned to each of the digital values which are represented by the time indication. For a typical clock that displays hours and minutes, such an arrangement requires twenty-seven signal carrying leads between the master and slave clocks, which obviously results in high cost for both initial installation and future maintenance.
It will be appreciated that typically, the slave clocks are specially designed and configured to operate in the master/slave environment. There are, however, commercially available digital clocks, comparatively inexpensive, intended to run autonomously. One such type is a commonly known battery operated or electric digital clock. When power is interrupted, the clock resets to 12:00 and continues flashing until it is manually reset.
Many commercially available digital clocks include a power up initialization circuit which sets the clock to a predetermined time, such as 12:00, upon the initial application of power. In addition, such clocks often have fast set and slow set manual controls for adjusting the time, or setting the minute and hour on the clock. For example, upon activation of the slow set manual control, the time is advanced at a rate such as one minute per half second, while the fast set control typically operates at a quicker rate. Those controls are available to allow a user to set the time manually after application of power.
Currently, one common digital clock requires two wires to operate and an additional two wires to reset. It has only one reset mode which is at 12:00 am. Twice in every twenty-four hour period, at noon and at midnight, the master clock resets the digital clock to 12:00. Such a clock requires a battery and flashes when the battery runs down and power failure occurs. Extra expense is needed for additional cabling, battery, battery connectors, and mounting clips. Extra labor is involved in pulling of the additional cabling and battery replacement when required. The end user will also receive incorrect time for a period of up to twelve hours if the battery fails and power is interrupted.
Another common digital clock is connected by data transmission lines and run by local power. This digital clock requires special data transmission equipment and noise immune cabling. This special data transmission line is expensive and costly to install. Locating power outlets means the installation point of the digital clock is costly, especially in retrofit applications.
Another master/slave clock system is disclosed in U.S. Pat. No. 4,490,050. In the '050 patent, each slave clock of the master/slave clock system includes an unregulated dc supply adapted to be supplied with ac power from the master clock. Unfortunately, only a limited number of slave clocks can be included in the system without adding one or more additional power supplies.
It is seen then that there exists a need for an efficient and inexpensive means of allowing a digital clock to be run and reset quickly to the correct time of day.